1. Field of the Invention
The present invention relates to electrophotographic photoreceptors having a photosensitive layer formed on an electroconductive substrate. More particularly, the present invention relates to electrophotographic photoreceptors which contain a resin for an electrophotographic photoreceptor in which the resins is excellent in solubility in preparation of a coating liquid to form a photosensitive layer and storage stability of the coating solution, and has good electric response characteristics.
2. Discussion of the Background
Electrophotographic technology, in recent years, has found widespread application not only in the field of copying machines, but also in the field of various printers, because it can quickly provide images of high quality.
As for the photoreceptor which is the core of the electrophotographic technology, there have been developed photoreceptors using, as the photoconductive materials, conventional inorganic photoconductors such as selenium, arsenic-selenium alloy, cadmium sulfide, and zinc oxide, and in recent years, organic photoconductive materials which have the advantages of entailing no pollution, ensuring easy film-forming, being easy to manufacture, and the like.
As the organic photoreceptors, there are known the so-called dispersion type photoreceptors obtained by dispersing a photoconductive fine powder in a binder resin, and the lamination type photoreceptors obtained by laminating a charge generation layer and a charge transport layer. Further, as the lamination type photoreceptor, an ordered lamination type photoreceptor wherein the charge generation layer and the charge transport layer are laminated in this order on an electroconductive substrate, and a reversed lamination type photoreceptor wherein the charge transport layer and the charge generation layer are laminated in this order, are known.
The lamination type photoreceptor has a high possibility of ranking as a dominant photoreceptor because: (1) a high sensitivity photoreceptor can be provided by using a charge generation material and a charge transport material each having a high efficiency in combination; (2) a high safety photoreceptor can be obtained because of its wide material selection range; (3) and it is relatively advantageous in terms of cost due to its high productivity of coating. Therefore, it has been vigorously developed and has gone into actual use.
The electrophotographic photoreceptor is repeatedly used in an electrophotographic process, i.e., in cycles of charging, exposure, development, transfer, cleaning, charge removal, and the like, during which it is subjected to various stresses to be deteriorated. Such deterioration includes chemical or electrical deterioration due to the following facts. That is, strongly oxidizing ozone or NOx arising from, for example, a corona charger commonly used as a charger can cause chemical damage to a photosensitive layer. In addition, carriers (current) generated upon image exposure passes through the inside of the photosensitive layer and can damage the photosensitive layer. The photosensitive composition may also be decomposed by charge-removed light or light from the outside. Further, there may occur mechanical deteriorations of abrasion or occurrence of flaws on the surface of the photosensitive layer, or peeling off of a film due to rubbing with a cleaning blade, a magnetic brush, or the like, contact with a developing agent or paper, and the like.
Especially, such damage on the photosensitive layer surface tends to become evident on the copied image. Accordingly, it directly damages the image quality and hence it is largely responsible for restricting the life of the photoreceptor. Accordingly, enhancement of the electrical and chemical durability of the photosensitive layer, as well as the enhancement of the mechanical strength of the photosensitive layer, are essential conditions for developing a long-life photoreceptor.
In general, it is a charge transport layer which is the outermost layer that receives such a load in the case of the ordered lamination type photoreceptor. The charge transport layer generally comprises a binder resin and a charge transport material. It is the binder resin that substantially determines the strength. However, since the amount of the charge transport material to be doped is considerably large, a sufficient mechanical strength has not yet been achieved.
Further, there has been a demand for a material adaptable to a higher-speed electrophotographic process to meet a growing need for a higher-speed printing. In this case, the photoreceptor is required not only to have a high sensitivity and a long life, but also to have good response characteristics so as to reduce the length of time between exposure and development thereof. It is known that, although the response characteristics are controlled by the charge transport layer, especially the charge transport material, it is also largely affected by the binder resin.
Each of the layers constituting the electrophotographic photoreceptor is formed by coating on a substrate by dip coating, spray coating, nozzle coating, bar coating, roll coating, blade coating, or the like.
Particularly, as the method of forming the charge transport layer, a known method of coating a coating solution obtained by dissolving materials to be contained in the layer in a solvent, for example, has been applied. In such an existing process, a coating solution is preliminarily prepared and preserved. Accordingly, the binder resin is required to be excellent also in regard to solubility in a solvent used for coating process, and stability of the coating solution after dissolution.
As conventional binder resins of the charge transport layer, there have been used thermoplastic resins and various thermosetting resins, including vinyl polymers such as polymethyl methacrylate, polystyrene, and polyvinyl chloride, and copolymers thereof, polycarbonate, polyester, polysulfone, phenoxy, epoxy, and silicone resins. The polycarbonate resin has a relatively excellent performance out of a large number of the binder resins, and hence various polycarbonate resins have been developed and have gone into actual use so far. However, in actuality, since the conventional organic photoreceptors employing conventional polycarbonate exhibit the drawbacks that the surface is worn and that flaws on the surface occur due to loads applied in use, such as development with toner, friction with paper, and abrasion by the cleaning member (blade), they have only the restricted printing performances in actual use.
On the other hand, in JP-A-56-135844, there is disclosed the technology of an electrophotographic photoreceptor using a polyarylate resin and a binder, commercially available under the tradename “U-polymer”. In the publication, it is shown that the electrophotographic photoreceptor thus disclosed is particularly excellent in sensitivity as compared with the one using polycarbonate.
In JP-A-3-6567, there is disclosed an electrophotographic photoreceptor containing a polyarylate copolymer having a structure using tetramethylbisphenol F and bisphenol A as bisphenol components.
Further, in JP-A-10-288845, it is disclosed that use of a polyarylate resin using a bisphenol component of a specific structure as the binder resin improves the solution stability in manufacturing the photoreceptor. In JP-A-10-288846, it is disclosed that an electrophotographic photoreceptor using a polyarylate resin having a specific kinematic viscosity range is excellent in the mechanical strength, especially the abrasion resistance.
However, when currently available polyarylate resins are used for the electrophotographic process, although abrasion resistance and sensitivity may be slightly improved, the stability of the coating liquid is poor, the coating production is difficult, and no adequate performance can be obtained in view of electric characteristics particularly sensitivity and response characteristics.
Further, although the solubility/solution stability, the mechanical strength, and the like are improved by using the polyarylate resin of a specific structure as disclosed in JP-A-10-288845 and JP-A-10-288846, the electric characteristics, especially the response characteristics have been unsatisfactory because of a recent growing demand for a higher-speed printing.
Therefore, in actuality, there has been a demand for a binder resin which ensures an excellent mechanical strength, is easy to dissolve in a coating solution solvent and is excellent in the storage stability of the coating solution, and is excellent in electric characteristics particularly response characteristics.
Further, in JP-A-57-73021 (U.S. Pat. No. 4,426,511, European Patent No. 0,050,821), a polyarylate of a specific structure excellent in heat resistance is disclosed. However, in the publication, application of such a polyarylate of the specific structure to electrophotographic photoreceptors, and required mechanical characteristics, electric characteristics, and the like, are not disclosed.
Thus, there remains a need for improved electrophotographic photoreceptors, which exhibit improved mechanical properties and electrical properties. There also remains a need for improved methods of producing such electrophotographic photoreceptors and improved materials for producing such electrophotographic photoreceptors.